Generally, the paper manufacturing process employs a machine that systematically de-waters a pulp slurry which consists largely of cellulose wood fibers, along with various chemical additives used as fillers and functional components of the paper or paper products. The pulp is prepared from various species of wood, by basically either of two pulping methods: chemical digestion to separate the cellulose fibers from lignin and other natural organic binders, or by mechanical grinding and refining. The resulting cellulose fibers are used in the manufacture of paper products whereby the pulp is supplied to a paper machine system, slurried in water to various solids levels (consistency), and ultimately diluted to about 0.5-1.0% solids for subsequent de-watering to form a sheet of paper. The low consistency of solids is necessary in order to facilitate fast drainage on the former while achieving proper fiber-to-fiber contact and orientation in the sheet. De-watering begins on the former, which is a synthetic wire or mesh that permits drainage to form a wet-web.
The web is then transferred into the machine press section and is squeezed between roller nips and synthetic press felts (predominantly comprised of nylon) to further remove water, and then through a dryer section comprised of steam-heated roller cans. Finally, the sheet is wound onto a reel. Other process stages can include on-machine surface sizing, coating, and/or calendaring to impart functional paper characteristics.
Generally, the wet-web is approximately 20% solids coming off of the former, 40% solids after leaving the press section, and about 94-97% solids (3-6% moisture) as the paper on the reel. Various chemical compounds are added to the fiber slurry to impart certain functional properties, to different types of paper. Fillers such as clay, talc, titanium dioxide, and calcium carbonate may be added to the slurry to impart opacity, improve brightness, improve sheet printing, substitute for more expensive fiber, improve sheet smoothness, and improve overall paper quality. Also, various organic compounds are added to the fiber slurry to further enhance paper characteristics. These include: sizing agents (either acid rosin, or alkaline AKD or ASA) to improve sheet printing so the ink doesn't bleed through the sheet, starch for internal fiber bonding strength, retention aids to help hold or bind the inorganic fillers and cellulose fines in the sheet, brightening compounds, dyes, etc. Therefore, as the sheet is de-watered on the paper machine, many types of deposits can result on the papermaking equipment. These deposits result from the chemicals used in the process, along with the natural wood compounds that are not thoroughly removed from pulping processes, or from inclusion of recycled fiber in the pulp slurry, and as a result of water re-use.
The primary function of the press-felt fabrics (other than a means of sheet conveyance) is to aid in the de-watering process of the wet-web. The press felts act like blotters or sponges that receive water that is expressed from the web by the pressure of the roller nips. On most modern paper machines, the water is then removed from the press felts by vacuum elements in the press, consisting of the Uhle boxes and suction press rolls. The press felts return in their travel loop back to the nip, to continually receive and transport water away from the web. Consequently, the press felts become contaminated with various types of soils resulting from the web compounds, and from the process shower waters used to flush the felts. Additionally, available chlorine is used in the treatment of paper machine press shower waters, which are used for felt washing and conditioning, in order to prevent microbial growths that result in slime formation that subsequently causes plugging of the shower nozzles. The residual chlorine, however, is detrimental to the nylon press fabrics. Over-treatment, or long-term accumulative effects of available chlorine can cause attack of the polyamide to the point where felt fiber shedding occurs, and press felt integrity is lost. Not only does this cause premature wear, and shorten the useful life of the press felt, but the fractured nylon fibers that become loosened from the felts contaminate the paper.
Additionally, if the paper is surface treated in the manufacturing process, i.e., on-machine coated or sized, these surface treatment systems become contaminated with the nylon-felt fibers, by transference. Sheet defects can become predominant, as manifested in “blade scratches”, when felt fibers are “snagged” by a blade coater.
Prior felt washing methods, used during the papermaking process, have relied upon dedicated chemical showers. There are four basic types of felt showers. Flooding showers are low pressure, high volume showers that flush loose particles and maintain the evenness of the water distribution in the felt. These are most effective at removing contaminants when used in conjunction with the nip of an inside felt carrying roll and require adequate vacuum to remove water volume. Flooding showers are used in tissue applications and on bleed-thru prone fine paper pickup felts.
Lubricating showers are low pressure, low volume shower used to apply a thin lubricating film of water to the felt prior to contact with a suction box to reduce wear and friction and act as a seal for the suction box. These showers apply a fan spray into the nip of the suction box with an overlapping coverage.
Chemical showers are low pressure, lower volume showers used to apply chemicals to the felt. These are most effective at removing contaminants when used in conjunction with the nip of an inside felt carrying roll. For maximum efficiency/dwell time, this shower should be placed as close to the sheet felt split and as far from the suction box as possible.
High Pressure showers are low volume showers used to physically dislodge contaminants from the felt. These are most efficient when placed close to a supporting roll.
High pressure cleaning of felts is best accomplished with an oscillating needle jet at controlled pressures. Proper oscillation of the high-pressure shower to assure uniform felt coverage is essential to an efficient felt conditioning system. Improper shower oscillation can result in a streaky felt appearance. Some sections of the felt do not receive showering and become filled while other sections of the felt receive partial or uniform showering.
All modern paper machine press sections are equipped with high pressure oscillating needle showers, just prior to the Uhle or vacuum box, as standard equipment from the machine manufacturer. These showers are provided as a means of mechanical cleaning, in order to both “chisel” away surface deposits and to loosen soils deep within the press felts void volume or base cloth. As an example, the oscillating needle showers may operate at pressures typically in the range of 150-250 psi, equipped with 0.040″ orifice spray nozzles, which are space 3″-6″ apart. These showers are designed to oscillate so as to allow the needle jets to cover the entire cross-machine direction of the press felt. The oscillation speed should ideally be matched to the rotation frequency of the press felt, so as to cover a cross-machine directional distance equal to the nozzle jet diameter, i.e., 0.040″, within the time of one nip rotation of the fabric (typically 2-4 seconds). Additionally, the shower oscillation stroke distance is often twice the needle-jet shower spacing, in order to obtain double full spray coverage of the felt. This is to compensate for a possible spray void area, should a nozzle become plugged.
Although chemicals have been applied to felts using high pressure showers at low part per million concentrations, these chemicals were limited to “conditioners” or preventative soil agents applied on a continuous basis. The high operating pressure of the needle poses difficulty in achieving sufficient cleaner concentrations to achieve adequate soil removal, so as to restore felt void-volume sufficiently, to improve felt permeability and water transport in a short period of time, such as 10-60 minutes per cleaning application. Applying sufficient cleaning composition to the felt on a continuous bases is cost prohibitive.
Further cleaning press fabrics “on-the-run”, while manufacturing paper, by injecting a detergent cleaner into the intrinsic high-pressure oscillating needle showers of a press felt, so as to remove papermaking soils for maintaining adequate press fabric de-watering, must be accomplished without adding water to the press, without disrupting the papermaking process (sheet breaks), and without causing off-quality product or sheet defects. Thus, high pressure showers have not been used for remedial or restorative chemical cleaning of press felt.